Scyllo-inositol (cis-1,3,5-trans-2,4,6-cyclohexanehexyl) is an optically inactive isomer of inositol and is a compound that was found long ago in plants and animals. Recently, however, various bioactivities of scyllo-inositol have drawn attention.
For example, Non-patent Reference 1 reports that scyllo-inositol has an inhibitory effect on amyloid β protein aggregation. This effect suggests the potential usefulness of scyllo-inositol in the treatment of Alzheimer's disease. Patent Reference 1 claims a blood sugar-lowering agent containing scyllo-inositol as an active ingredient. Therefore, there clearly exists a need to industrially produce scyllo-inositol.
Classic production methods were extraction of scyllo-inositol from plants or chemical synthesis of this compound using myo-inositol as a raw material (Non-patent References 2 and 3, Patent Reference 2, and the like). In recent years, however, more efficient methods of producing scyllo-inositol using natural microorganisms or enzymes from microorganism have been studied.
Patent Reference 3 discloses a method for producing inositol stereoisomers in culture broth by culturing microorganisms belonging to the genus Agrobacterium in medium containing myo-inositol or producing inositol stereoisomers by causing cells or treated cells of microorganisms belonging to the genus Agrobacterium to act on myo-inositol. These isomerizations are said to convert myo-inositol into a mixture of scyllo-inositol, chiro-inositol (as a mixture of D- and L-forms), and neo-inositol.
Patent Reference 4 states that myo-inositol is converted into scyllo-inosose by causing Pseudomonas sp. AB10064 (FERM P-18330) or Acetobacter sp. AB10253 (FERM P-18868) to act on myo-inositol. Synthesis of scyllo-inositol by reducing the scyllo-inosose produced in this way by sodium borohydride was also attempted, but this reduction treatment basically produced scyllo-inositol only as a mixture with myo-inositol (that is, a retrograde reaction to the raw material). Therefore, it was necessary to increase the content of scyllo-inositol gradually while repeating conversion of myo-inositol into scyllo-inositol by microorganisms and reduction treatment by sodium borohydride in the method for producing scyllo-inositol described in Patent Reference 4.
Patent Reference 5 discloses a method for producing scyllo-inositol using myo-inositol as a raw material, in which myo-inositol is enzymatically converted into scyllo-inositol in a solution obtained by mixing myo-inositol 2-dehydrogenase (EC 1.1.1.18) which produces scyllo-inosose from myo-inositol, scyllo-inositol dehydrogenase which stereoselectively reduces scyllo-inosose to scyllo-inositol, and NAD+ or NADP+. The conversion of myo-inositol into scyllo-inositol is said to be 31% on a yield base in this reference.
Therefore, all of the above references relate to methods for producing scyllo-inositol using myo-inositol as a raw material; none teach the de novo biosynthesis of scyllo-inositol, that is, direct production of scyllo-inositol from ubiquitous raw materials such as glucose and the like by a one-step process.
In particular, myo-inositol itself is in the first place an extremely useful and valuable bioactive substance. Specifically, myo-inositol is widely utilized as a component of nutritional foods, feeds, pharmaceuticals, and the like since it is an essential substance for many higher animals. For example, myo-inositol is known to play an important role in the metabolism of fats and cholesterols and is held to be effective in the prevention and treatment of hypercholesterolemia and the like.
Therefore, many improvements are in fact being proposed for industrial-scale myo-inositol production processes. For example, Patent Reference 6 discloses the discovery and utilization of yeast of the genus Candida capable of secreting inositol extracellularly. Patent References 7 and 8 disclose the introduction of mutations to impart resistance to glucose antimetabolites and antibiotics, respectively, to the above yeast of the genus Candida. Patent References 9, 10, and 11 also disclose improvement of the yield of inositol by introducing mutations to impart resistance to tertiary amines, hexachlorocyclohexane, and cetyl trimethylammonium salt, respectively, to yeasts of the genus Candida having the ability to produce inositol. Patent Reference 12 discloses the introduction of a mutation to impart resistance to 6-halogeno-6-deoxyglucose to a yeast of the genus Candida having the ability to produce inositol. Patent Reference 13 also discloses the introduction of a mutation to impart resistance to halogenated pyruvic acid to a yeast of the genus Candida having the ability to produce inositol. In addition, Patent Reference 14 discloses that it is possible to impart the ability to produce inositol to a yeast of the genus Candida that does not have the ability to secrete inositol by transforming the yeast by inositol-1-phosphoric acid synthase-encoding DNA alone, based on the reasonable inference that inositol-1-phosphoric acid synthase is responsible for a rate-limiting reaction in the series of myo-inositol biosynthetic reactions. Patent Reference 15 discloses that the inositol productivity of the yeast is improved by introducing inositol-1-phosphoric acid synthase-encoding DNA alone into yeast under the control of a glycerol-3-phosphate dehydrogenase gene promoter.
All of the above tells us that establishing an efficient, economical production method for myo-inositol itself still remains a significant technical problem even today. Therefore, the scyllo-inositol production processes of the prior art that must use valuable, expensive myo-inositol as a raw material are obviously inefficient and uneconomical.
Moreover, none of the above references disclose or even suggest a scyllo-inositol derivative, especially scyllo-inositol derivatized from sugars.